1. Field of the Invention
The present invention relates to a variable gain amplifier (VGA), and more particularly, to a complementary metal oxide semiconductor (CMOS) variable gain amplifier having a wider variable gain range than a conventional VGA in the same control voltage variation range.
2. Discussion of Related Art
In general, in a radio frequency (RF) communication transceiver, a receiver has a variable gain amplifier for producing, according to the magnitude of an input signal, a constant signal-to-noise (SNR) characteristic. The variable gain amplifier allows the input signal, whose magnitude in decibels (dB) varies with a distance, to fix the magnitude of a final analog signal of the receiver. Further, the variable gain amplifier serves to provide decibel-linear gain variation with a control voltage in order to make a time taken to perform automatic gain control (AGC) uniform.
An example of the conventional variable gain amplifier is a variable gain amplifier using a bipolar junction transistor (BJT). Owing to the BJT, the variable gain amplifier represents an exponential characteristic of output current depending on input voltage and accordingly, has the advantage of easily obtaining linear-in-dB variable gain.
Another example of the conventional variable gain amplifier is a variable gain amplifier using a complementary metal oxide semiconductor (CMOS). The CMOS variable gain amplifier represents a square law characteristic in which radio frequency input power from a noise level to a −20 dB level is proportional to the square of small signal voltage. Therefore, the CMOS variable gain amplifier generally has a circuit for obtaining a pseudo decibel-linear gain characteristic. However, unlike the variable gain amplifier using the BJT, the CMOS variable gain amplifier has many transistors, and thus has a complicated circuit, high current consumption, a large noise factor, and a considerably large design area.